1. Field of the Invention
This invention relates to a sewing machine control device for controlling the number of rotations a sewing machine for a given sewing operation so as to maintain a correct, predetermined number.
2. Background of the Invention
Generally, many electro-motive type sewing machines driven by motors are used for industrial sewing machines. In the sewing machine of this type, the rotation of the motor is transferred to the shaft of the sewing machine by means of pulleys and a belt so as to rotate and drive the shaft. Published unexamined Japanese Patent Application No. 33979/1983 discloses a conventional sewing machine control device, in which an alternating current motor is used as the motor, and the number of rotations of the motor is changed by a method for controlling the frequency of the electric power supplied to the motor or by other methods in order to control the number of rotations of the shaft of the sewing machine.
In this prior art open-loop type sewing machine in which the number of rotations of the sewing machine is not fed back to the motor drive controller, however, the number of rotations of the motor is set to a predetermined number in order to control the number of rotations of the sewing machine with a constant rotation transfer ratio being assumed. Therefore, the prior art includes the problem that the sewing machine is not correctly controlled when the pulleys are changed and the rotation transfer ratio is changed.